1. Field of the Invention
The invention relates to a novel optical biosensing platform in general, and more particularly to an optical biosensing platform utilizing surface-modified nanocrystalline zinc oxide for the real-time detection of the occurrence of a binding event.
2. Description of the Prior Art
There are various optical biosensing platforms which are currently used to identify the occurrence of a binding event between biomolecules, e.g., between a receptor and a ligand.
One of these optical biosensing platforms combines the use of biomolecules and a biosensing substrate (e.g., a high surface area material), wherein the biosensing substrate is used to provide an optical signal resulting from the binding event. For example, porous silicon cast films have been used as biosensing substrates because of their ease of fabrication, inherent optical properties, porosity, and the ability to chemically functionalize silicon. More particularly, the porosity of the silicon substrate increases the available surface-area for the attachment of receptors.
The surface of porous silicon cast films can be chemically-derivatized to introduce various reactive groups (e.g., —NH2, —COOH, —SH, etc.) in order to enable covalent immobilization of biomolecules. These immobilization techniques retain the activity and long-term stability of the biomolecules. By way of example but not limitation, biomolecules, such as enzymes, DNA, proteins, cells, and lipopolysaccharide components, etc. have all been successfully immobilized to porous silicon for use in the capture of specific analytes. When using porous silicon substrates, the occurrence of specific binding events is measured and/or identified by variations in refractive index, shifts in reflectance, ellipsometry, and/or photoluminescence, etc.
While porous silicon may be used as an optical biosensing platform, its use is hindered by the need to tailor the pore size of the silicon in order to accommodate a specific receptor-ligand pair of interest. In addition to the time required to prepare a specific pore size and platform for each receptor-ligand pair of interest, the steric hindrance imposed by the porous structure limits the usable surface area available for receptor-ligand pair binding. Together, these disadvantages significantly complicate the process of using porous silicon as an optical biosensing platform.
In addition to silicon substrate platforms, other techniques exist for optically detecting binding events. Some optical biosensing techniques, such as those associated with magnetic nanoparticles, utilize an indirect approach for detecting binding events. These techniques require labeling one or more of the biomolecules (e.g., receptors, ligands, etc.) with a fluorescent “reporter” tag molecule. Subsequently, the “tagged” biomolecule is passed through an appropriate optical reader in order to detect whether a binding event has occurred.
These techniques have numerous disadvantages including increased complexity, lengthy sample preparation techniques, time-consuming analysis, and limited sensitivity.
Accordingly, there is a need for an optical biosensing platform wherein the pore size does not need to be tailored for a specific receptor-ligand pair and which also overcomes the limitations associated with fluorescent tagging.
Nano-ZnO has the desirable qualities indicated above, e.g., large surface area, mechanical and thermal stability, and an inherent photoluminescence signal.
Nano-ZnO is presently used as a wide band gap semiconductor due to its potential applications in the areas of photonics, electronics and sensors. For example, nano-ZnO has been used as a gas sensor by monitoring changes in its electrical resistivity.
In addition, nano-ZnO has been used as a biosensor platform wherein the binding of a target analyte is detected using a variety of techniques. These include obtaining electrical measurements, monitoring changes in conductivity, using quantum dots, incorporating optical dyes and/or measuring changes in the optical density of ZnO.
The photoluminescence signal inherent to as-grown ZnO nanostructures (FIG. 1) consists of two emission peaks. One of the peaks is emitted within the ultraviolet (UV) region and the other peak is emitted within the visible region of the electromagnetic spectrum (FIG. 2). The presence of these two distinct photoluminescent (PL) emission bands (i.e., ultraviolet and visible) make it desirable to use nano-ZnO as a potential real-time optical biosensing platform. More particularly, a surface binding event induces a change (e.g., in emission intensities, in emission maxima shifts and/or in peak proportionalities, etc.) within the inherent photoluminescent (PL) properties of nano-ZnO. This change can then be used to detect the binding event of a specific target ligand to the surface of the nano-ZnO. Thus, nano-ZnO eliminates the need for fluorescent labeling and provides an opportunity to detect real-time binding events through UV or visible peak emission intensity changes, emission-maximum shifts, and peak proportionality changes.
However, in order to detect the binding of a specific target ligand, the surface of the nano-ZnO must first be functionalized with an appropriate receptor. Moreover, through the process of functionalizing the nano-ZnO surface, it is possible that the inherent photoluminescent (PL) properties of nano-ZnO may be adversely affected. By way of example, previous studies have shown that surface alterations to nano-ZnO generally stabilize the ultraviolet (UV) emission but typically diminish the visible emission. This results in the loss of the versatility of the two distinct inherent emission peaks of nano-ZnO, and functionality as an optical biosensor platform is thereby significantly decreased. Furthermore, those surface-altered ZnO nanostructures have not introduced any level of chemical functionality and, therefore, are not satisfactory techniques in the development of nano-ZnO-based optical biosensors.
There is thus a need for an optical biosensor platform which remedies the aforementioned shortcomings of the current platforms in use.